1. Field of the Invention
The present invention relates to a device (hereinafter referred to as a light-emitting device) that has an element (hereinafter referred to as a light-emitting element) where a thin film including a luminescent material is sandwiched between a pair of an anode electrode and a cathode electrode. In particular, the present invention relates to a light-emitting device whose light-emitting element includes a thin film (hereinafter referred to as a light-emitting layer) made of an electro-luminescent material (EL material). The present invention also relates to a display device that uses a substrate made of an organic resin material and, more particularly, to a display device where a pixel portion is formed on such a substrate using thin-film transistors and an EL material.
2. Description of the Related Art
Liquid crystal panels or EL materials applied to display devices may contribute to reduction in weight and thickness thereof in comparison with conventional CRTs. Therefore, attempts have been recently made to apply display devices using the liquid crystal panels or EL materials to various fields. Also, it has now become possible to connect portable telephones and personal digital assistants (PDAs) to the Internet, which leads to the dramatic increase in the amount of image information to be displayed thereon and creates increasing demand for high-definition color display devices.
Display devices used for such portable information terminals need to be reduced in weight and, for instance, portable telephones whose weights are below 70 g are now on the market. For the reduction in weight, almost all components, such as electronic components, housing, and batteries, of the portable information terminals are subjected to reengineering. For the further weight reduction, however, display devices need to be reduced in weight.
Display devices are produced using glass substrates in many cases, so that one conceivable method for weight reduction would be to reduce the thickness of the glass substrates. In this case, however, the glass substrates tend to be cracked and the shock resistance thereof is lowered. This becomes a serious hindrance to the application of display devices including such thin glass substrates to portable information terminals. To meet demand for weight reduction as well as shock resistance, the development of display devices using organic resin substrates (plastic substrates) is under consideration.
For instance, light-emitting devices that have light-emitting elements produced using EL materials are currently under development. Display devices whose pixel portions are formed using light-emitting elements are capable of emitting light by themselves and further do not require light sources, such as backlights, unlike liquid crystal display devices. As a result, such light-emitting elements are highly expected as an effective means for reducing weights as well as thickness of display devices.
The construction of a typical light-emitting element using an organic EL material is shown in FIG. 22. In this drawing, an insulator 2201, an anode 2202, a light-emitting layer 2203, and a cathode 2204 are laminated to form a light-emitting element 2200.
Before being observed by an observer 2206, light 2205 emitted from the light-emitting layer directly passes through the anode 2202, or is reflected by the cathode 2204 and then passes through the anode 2202. That is, the observer 2206 observes the light 2205 that and passes through the anode 2202 to be emitted in picture elements where the light-emitting layer 2203 performs light emission.
A light-emitting element is composed of two electrodes: an anode that injects holes into an organic compound layer including a light-emitting layer, and a cathode that injects electrons into the organic compound layer. The light-emitting element having this construction utilizes a phenomenon where light is emitted when the holes injected from the anode are recombined with the electrons injected from the cathode within the light-emitting layer. The organic compound layer including the light-emitting layer is degraded by various factors, such as heat, light, moisture, and oxygen. To prevent this degradation, an ordinary active matrix type light-emitting device is produced by forming light-emitting elements in a pixel portion after wiring and semiconductor elements are formed therein.
After the formation of the light-emitting element, a first substrate, on which the light-emitting element have been formed, and a second substrate for covering the light-emitting elements are laminated and sealed (packaged) using a sealing member. This construction prevents the light-emitting elements from being exposed to the outside air.
It should be noted here that in this specification, all layers provided between a cathode and an anode are collectively referred to as an organic compound layer. The organic compound layer has a well-known structure where, for instance, a hole injecting layer, a light-emitting layer, an electron transporting layer, and an electron injecting layer are laminated with each other. A predetermined voltage is applied to the organic compound layer by a pair of electrodes to cause the recombination of carriers, thereby causing light emission in the light-emitting layer.
The light-emitting element, however, has a problem as to durability and, in particular, to oxidation resistance. The cathode that injects electrons into the organic compound layer is ordinarily made of an alkaline metal or an alkaline earth metal having a low work function. It is well known that these metals tend to react with and water, thereby having low oxidation resistance. The oxidation of the cathode means that the material of the cathode loses electrons and is coated with an oxidation layer. The reduction in the number of electrons to be injected and the oxidation coat may reduce the amount of emitted light in brightness.
As described above, the electrode of the light-emitting element is easily oxidized with a considerably small amount of oxygen or moisture and therefore the light-emitting element is easily degraded. Various techniques have been developed to prevent the oxidation of the light-emitting element. For instance, the light-emitting element is sealed with a metal or glass that is impermeable to oxygen and moisture. Also, the light-emitting element is produced to have a resin lamination construction or is filled with nitrogen or an inert gas. Even if the light-emitting element is sealed with a metal or a resin, however, oxygen easily passes through small gaps and oxidizes the cathode and light-emitting layer. Also, moisture easily passes through the resin used to seal the light-emitting element in terms of the light-emitting element. This causes a problem in that areas (called dark spots) that do not emit light appear on a display screen and expand with the lapse of time, which makes the light-emitting element incapable of emitting light.
EL materials are capable of emitting blue light and thus it is possible to realize a full-color display device of a self-light emitting type with the materials. However, it is confirmed that organic light-emitting elements are degraded in various ways. This degradation prevents the actual use of the EL materials and a solution to this problem is urgently required. The dark spots are spot-shaped defects that do not emit light in the pixel portion and so degrade display quality. The dark spots are also defects that get worse over time. Even if the light-emitting element is not brought into operation, the number of the dark spots is increased by the existence of moisture. It is thought that the cause of the dark spots is the oxidation reaction of the cathode made of an alkaline metal. To prevent the occurrence of dark spots, a sealed space is filled with dryer gas or provided with a dryer agent, in which the light-emitting element is placed.
Also, the light-emitting element is vulnerable to heat that promotes oxidation. This means that there are many factors causing oxidation and therefore it is difficult to make actual use of light-emitting devices. In view of the problems described above, the object of the present invention is to provide a light-emitting device with a high degree of reliability and an electronic device where a high-reliability display unit is achieved using such a light-emitting device.
It is well known that a substrate made of an organic resin material has high permeability to moisture, in comparison with a glass substrate. For instance, the permeability to moisture of polyether imide is 36.5 g/m2xc2x724 hr, that of polyimide is 32.7 g/m2xc2x724 hr, and that of polyether terephthalate (PET) is 12.1 g/m2xc2x724 hr.
As is apparent from this, if a display device produced with a light-emitting element including an organic resin substrate is left standing in the air for a long time period, moisture gradually permeates and the organic light-emitting element is degraded. In addition, a sealing member used to seal a light-emitting element is also made of an organic resin material, so that it is difficult to completely prevent oxygen and moisture in the air from entering through sealed portions.
Also, an organic resin substrate is soft, in comparison with a metal substrate or a glass substrate, so that scratches or the like are easily made thereon. Further, the long-term exposure to the direct sunlight causes a light chemical reaction and alters the quality and color of the organic resin substrate.
As described above, the organic resin substrate is a highly effective means to realize a display device reduced in weight with high shock resistance, although there remain many problems that must be solved in order to ensure the reliability of the light-emitting element. In view of these problems, the object of the present invention is to provide a display device that uses a light-emitting element with a high degree of reliability.
Also, if the outside light (the light existing outside the light-emitting device) enters picture elements that do not emit light, the light is reflected by the back surface (the surface contacting the organic compound layer) of the cathode, so that the cathode back surface functions as a mirror and reflects the outside scenes. To solve this problem, a circular polarizing film has conventionally been applied to a light-emitting device to prevent the reflection of the outside scenes toward the observer, although this construction raises the fabrication cost because the circular polarizing film is high-priced. In view of this problem, the object of the present invention is to prevent this mirror reflection phenomenon of a light-emitting device without using a circular polarizing film.
According to the present invention, in a display device using an organic resin substrate, a hard carbon film is formed on a surface of the substrate as a protecting film that prevents from entering moisture or the like and the scratches on the surface. In particular, a DLC (Diamond like Carbon) film is used with the present invention. The DLC film has a construction where carbon atoms are bonded into a diamond bond (Sp3 bond) in terms of a short-distance order, although the film has an amorphous construction containing a graphite bond (Sp2 bond) from a macroscopic viewpoint. The DLC film contains 95 to 70 atomic % carbon and 5 to 30 atomic % hydrogen, so that the DLC film is very hard and excels in insulation. The DLC film is also characterized by low gas permeability to moisture and oxygen. Further, it is known that the hardness of the DLC film is 15 to 25 Gpa in the case of measurement using a micro-hardness meter.
The DLC film is formed using a plasma CVD method, a microwave CVD method, an electron cyclotron resonance (ECR) CVD method, or a sputtering method. With any of these methods, the DLC film is formed in intimate contact without heating the organic resin substrate. The DLC film is formed under a situation where the substrate is set on a cathode. Alternatively, the DLC film is formed by applying a negative bias and utilizing ion bombardment to some extent. In the latter case, the DLC film becomes minute and hard.
The reaction gas used to form the DLC film is hydrocarbon gas, such as CH4, C2H2, and C6H6. The DLC film is formed by ionizing the reaction gas by means of glow discharge and bombarding a cathode, to which a negative self-bias is applied, with accelerated ions. In this manner, the DLC film becomes minute and flat. The DLC film may be formed without heating the substrate to a high temperature, so that the formation of the DLC film can be performed in the final manufacturing step where a display device is finished.
By forming the DLC film on at least one surface of the organic resin substrate, the gas barrier property is improved. Alternatively, the gas barrier property is improved by forming the DLC film on the outer surface of a sealing member used to laminate an organic resin substrate (hereinafter, an element substrate), on which TFTs and light-emitting elements are formed, with a sealing substrate for sealing the light-emitting elements. In this case, the thickness of the DLC film is in a range of 5 nm to 500 nm. Also, by forming the DLC film on a light incident surface, ultraviolet rays are blocked, the light chemical reaction of the organic resin substrate is suppressed, and the degradation of the organic resin substrate is prevented.
The DLC film that prevents oxygen and moisture from entering is formed to successively cover exposed portions of the sealing member and side portions of the first and second substrates that are laminated to produce the light-emitting device. The exposed portions of the sealing member and the side portions of the first and second substrates are hereinafter collectively referred to as xe2x80x9cend surfacesxe2x80x9d. With a conventional technique, oxygen and moisture pass through a resin provided at end portions. The construction described above, however, prevents moisture from entering through between the first and second substrates.
A dryer agent is provided in a space between the element substrate and the sealing substrate sealed by the sealing member, thereby suppressing the degradation of the light-emitting elements. For instance, a barium oxide can be used as the dryer agent. The dryer agent is provided at positions (for instance, on a driving circuit, on a partition wall, or within the partition wall) outside light-emitting areas. With this construction, the dryer agent absorbs gas and moisture contained in the light-emitting elements as well as oxygen and moisture passing through a sealing resin in the end portions. As a result, the degradation of the light-emitting elements is prevented. Further, by forming an organic interlayer insulating film using a black resin, the mirror reflection phenomenon (the reflection of the outside scenes) of the light-emitting device is prevented. Also, the black resin may be used in an area in which the sealing member is formed.
The DLC film described above is applicable to passive type display devices as well as active matrix type display devices.